Single melt filters are used on the one hand where a continuous film production is not possible or not necessary so that an interruption of the production due to changing of filter cartridge or other maintenance work is acceptable. Such single melt filters are also used in a second filtering stage with a fine filtering cartridge downstream of a double melt filter system. Customarily the double melt filter is positioned between a melt pump and a film laminating extruder or a melt pump and a melt film extruder. The single melt filter is positioned downstream of the melt pump as close as possible to the extrusion nozzle. Downstream of the extrusion nozzle normally a cooling roller is arranged which cools the extrudate to form the film which then proceeds into a stretching system for further treatment.
Large surface area melt filters include conventionally a filter housing for holding an exchangeable filter cartridge. Double filters normally include melt valves arranged between two filter housings. The filter cartridges which are equipped with filtering elements must be removed from the filter housing for cleaning, whereby the filter housing remains in a fixed position in which it is heated either electrically or by a hot liquid or by steam. In order to assure an extrusion operation free of leakage, it is necessary that the melt inlets and outlets of the filter housings and their cartridges are pressed with relatively large forces into or against centering or sealing surfaces which contact the inlet and outlet of the filter housing. In order to produce the required high press-on forces, large dimension pressure screws or wedge-shaped rapid coupling connector are used. Due to the large forces and their repeated application and release, the centering and sealing surfaces must satisfy special conditions. To be able to satisfy these conditions it is necessary to carefully clean these surfaces at each cartridge exchange. Furthermore, it is necessary to carefully clean the melt inlets and outlets of the filter housings that are installed in a fixed position while a filter cartridge is withdrawn from a filter housing and then cleaned in a special room prepared or equipped for such cleaning operation.
It is a disadvantage of conventional filter devices for extruders that the space available around the inlets and outlets of the filter housings, is rather inadequate for the above mentioned cleaning operations. This lack of accessibility is aggravated by the fact that the operating temperature of the extrudate is about 800.degree. C. which requires a special dexterity of the maintenance personnel which must work under adverse conditions due to high heat radiation.
Furthermore, no damage may be caused to the surfaces that come into contact with the melt, particularly the sealing surfaces between the filter housing inlet and outlet and the valves. Avoiding such mechanical damage during cleaning and maintenance work is difficult under the above mentioned working conditions. Further, not properly cleaned melt channels can contaminate the extrudate when the filter is restarted after a cleaning operation. Such contamination of the melt in turn results in a faulty film product that can lead to tearing of the extruded film and the like.
Efforts have been made heretofore to increase the accessibility for cleaning purposes, whereby a conventional double filter is so constructed that both melt valves, the inlet valve and the outlet valve, are positioned separate from the filter housings to which the valves are connected through pipelines. The two filter housings are spaced from each other, just as the valves are. Such a construction makes it possible to supply melt to the first filter through a three-way valve and a pipeline until the first filter requires replacement or cleaning of its cartridge. The melt discharge pipe downstream of the filter is also a respective pipeline with a three-way discharge valve. When it becomes necessary to clean the first filter, the two inlet and outlet three-way valves are switched over in a determined, synchronized sequence for continuing filtration of the melt through the second filter. When the switch-over of the two three-way valves is completed, the melt flows through the inlet three-way valve with a respective pipeline through the second filter with its filter cartridge and through a further pipeline to the three-way outlet or discharge valve.
The just described conventional construction has its advantages as far as accessibility for cleaning purposes is concerned. However, a substantial structural volume with pipelines and three-way valves requires a correspondingly substantial space. Moreover, at least four pipelines with at least eight couplings requiring respective sealing devices must be cleaned. The filter must be mounted displaceable in the system in order to remove the pipelines from the system and to make the pipe couplings accessible for the cleaning. This requirement calls for an additional space and entails higher manufacturing, assembly and maintenance costs. The pipeline assembly for connecting the two three-way valves with the two filters entail unavoidable detouring of the extrusion melt, since the melt does not flow along a straight conduit. Hence, the feeding or transportation of the melt requires additional costs as far as energy consumption by the conveying pump and heating of the system are concerned.
Another conventional double filter system has a more compact assembly with but one valve having a double valve plug between the two filters, whereby the two filters are linearly displaceable in the system. Such a construction avoids the disadvantage of long pipelines and many pipe couplings, yet it requires, in spite of the more compact construction, a substantial space around the system because the filter housings with the filter cartridges must be displaced away from the two-way valve for cleaning of the couplings and the filter cartridges. During the cleaning operation the operating personnel is exposed to substantial thermal effects because the operator must perform his job by standing between the displaced hot filter housing and the remaining hot sections of the system. In addition to high manufacturing and construction or assembly costs, the operating costs are also contributing to a substantial disadvantage. A total of six melt detours in the piping system further contributes to the increased costs especially in terms of a higher power consumption.
Swiss Patent CH 487,730 (Zink) discloses a melt filter with two filter chambers that are tiltable or displaceable circumferentially about a mounting body that is provided with melt passages, whereby separate inlet and outlet melt valves are avoided. The mounting body itself is provided with melt passages that can be aligned with one or the other filter housing thereby providing the required valve functions. Each housing carries an exchangeable filter cartridge. Thus, by tilting the filter housings about the central axis of the mounting body, one or the other filter housing can be brought into a position for cooperation with an extruder and with a mold. For this purpose each filter housing is equipped with an inlet section connectable to the extruder and with an outlet section connectable to the mold, whereby the inlet and outlet sections provided respective melt inlet and melt outlet channels. Such a construction has the advantage that separate melt inlet and outlet valves are avoided and that one filter cartridge may be exchanged while the other filter cartridge in its housing is in a working position. However, maintenance and cleaning operations are aggravated because of the limited space available in the inlet and outlet area of the conventional filter system according to the Swiss Patent and due to the fact that a high radiation heat from the filter housing that is in a working position aggravates the working conditions. Thus, a proper maintenance and thorough cleaning operation is only possible when the melt flow is completely interrupted and the filter mounting body is tilted or shifted into the maintenance position with both filter housings.